1. Technical Field
Features described herein relate to an image forming apparatus for forming an image on a printing medium.
2. Related Art
A color ink-jet printer is known as an imaging forming apparatus. In this printer, ink droplets are ejected sequentially to a printing medium while an ink head for ejecting plural ink droplets is moved in the main scanning direction. After a scan in the main scanning direction has finished, an auxiliary scan is performed by, for example, moving the printing medium in a direction that crosses (e.g., in a direction that is perpendicular to) the main scanning direction and then a main scan is performed again.
In the ink head, discharge apertures (printing elements) for ejecting ink droplets are arranged in the auxiliary-scanning direction. From a main scan, these apertures eject ink droplets onto a printing medium, forming rows of plural dots arranged in the main scanning direction (sometimes referred to as “rasters”). When the printing medium is thereafter moved in the auxiliary-scanning direction by a ink head length, there may occur a phenomenon that the interval between the tail raster formed by the preceding main scan and the head raster formed by the current main scan becomes wider than the interval between rasters that are formed by one main scan due to, for example, an error of a feed mechanism for feeding the printing medium. In this case, white streaks occur which are called banding.
FIG. 10A shows example banding that occurs in the above manner. An ink-jet head 70 is formed with 100 discharge apertures that are arranged in the auxiliary-scanning direction. Dots indicated by white circles are formed by a main scan (preceding main scan) by the 97th to 100th discharge apertures of the ink-jet head 70 (the depiction of such dots herein assumes, of course, that the image being printed called for dots to be printed at those positions).
Then, after the printing medium has been transported in the auxiliary-scanning direction, dots indicated by black circles are formed by the 1st to 4th discharge apertures of the ink head 70. The printing medium should be transported by 101×d by the auxiliary scan, where d is the pitch of the discharge apertures of the ink-jet head 70. However, assume that the printing medium has been transported excessively (excess distance: Δ). In this case, the distance between the raster formed by the 100th discharge aperture in the preceding main scan and the raster formed by the 1st discharge aperture in the current main scan is equal to d+Δ. The raster interval is increased there to cause banding.
In one known method of reducing such banding, shown in FIG. 10B, the printing area of a preceding main scan and that of a current main scan partly overlap each other, so that for the rasters in the overlap area, some of the dots are printed by the preceding main scan, and the remaining dots are printed by the current main scan.
In the example of FIG. 10B, the rasters of the overlap area are formed by the 99th and 100th discharge apertures of the ink head 70 in the preceding main scan and by the 1st and 2nd discharge apertures of the ink head 70 in the current main scan.
FIG. 10C shows a case of the FIG. 10B technique, in which the printing medium was transported excessively (excess distance: Δ) when it was transported in the auxiliary-scanning direction. Dots formed by the 1st discharge aperture of the ink head 70 are deviated downstream by Δ from dots formed by the 99th discharge aperture of the ink head 70, and dots formed by the 2nd discharge aperture of the ink head 70 are deviated downstream by Δ from dots formed by the 100th discharge aperture of the ink head 70. However, since the gaps caused by these deviations are not located on straight lines, they are less noticeable, and the degree of banding can be reduced.
In the technique discussed above, one raster is printed by plural main scans. If an error occurs between a preceding main scan and a current main scan, dots formed by the preceding main scan and dots formed by the current main scan may overlap with each other. Such overlapping dots may be more noticeable to the human eye, and is unacceptable in view of increasing demand for higher print accuracy. Improving print accuracy is also hampered by the increasing demand for higher print speeds. As the movement speed in the main scanning direction is increased, the shape of the dots becomes less of a circle, and more like an ellipse that is long in the main scanning direction. As the print speed is increased, the time interval between dot formation by a preceding main scan and that by a current main scan (i.e., the time from landing of preceding ink droplets onto a printing sheet to landing of current ink droplets) becomes shorter, and accordingly the dot movement distance from the preceding main scan to the current main scan becomes shorter. Therefore, if a deviation occurs in the main scanning direction, dots formed by the preceding main scan and dots formed by the current main scan become more prone to overlap with each other. When a dot of the current main scan is superimposed on a dot of the preceding main scan before the latter dries, then the two dots are combined into a single dot having a larger diameter. When such combined dots have larger diameters, a user looking at a resulting image formed on the printing sheet may feel that the image has a portion where dots (grains) are conspicuous. That is, the accuracy of the dots is deteriorated. FIG. 10D shows how elliptical dots formed by a preceding main scan and elliptical dots formed by a current main scan are connected to each other. If dots are connected to each other in this manner, small dots are particularly deteriorated in appearance when combined into larger dots. This means a problem that a rough, grainy image is formed instead of an intended high-resolution image.